by timothy.stranex:

$ go version
go version go1.2 darwin/amd64

Here is a link to a program on play.golang.org that demonstrates the problem.
http://play.golang.org/p/HXNEgYhZgE

For reference, this is the program:

-------
package main

import "encoding/json"
import "fmt"

type DecimalE8 int64

func (d *DecimalE8) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf("%.8f", float64(*d) / 1e8)), nil
}

type Payment struct {
    Amount DecimalE8
}

func main() {
    var p Payment
    p.Amount = 123456789

    s1, _ := json.Marshal(p)
    fmt.Println(string(s1))

    s2, _ := json.Marshal(&p)
    fmt.Println(string(s2))
}
-------

The actual output from this program is:
{"Amount":123456789}
{"Amount":1.23456789}

The expected output is:
{"Amount":1.23456789}
{"Amount":1.23456789}

Basically it seems that DecimalE8.MarshalJSON isn't being used when the struct is passed
as a value to json.Marshal but it is used when it's passed as a pointer. I would expect
that DecimalE8.MarshalJSON be used in both cases.